Engine control systems often determine the amount of fuel to inject by measuring a manifold pressure, along with other engine operating conditions. This method is often referred to by those skilled in the art as the speed density method. In this method, a mean value model of engine operation is constructed, where an average manifold pressure at a given speed results in a certain air flow into the cylinder. In this type of system, measurement of the manifold pressure is critical for proper prediction of the air flow into the cylinder and thus for proper air/fuel ratio control.
One approach for calculating a value of the manifold pressure to use in the speed density approach is to sample the manifold pressure sensor when the piston is at top dead center, bottom dead center, and two other points equally spaced from dead center positions. For example, the two other equally spaced samples could be 60 degrees after dead center. Thus, four samples per revolution of the crankshaft are used that are not necessarily equally spaced. Then, an average of the last two and the current value of the manifold pressure is taken to obtain the averaged value of the manifold pressure used in the speed density method. Such a system is disclosed in U.S. Pat. No. 5,497,329.
The inventors herein have recognized numerous disadvantages with the above approach. For example, the sampling scheme described above will produce a constant bias unless the two equally spaced samples occur in the proper location. In particular, the resulting averaged manifold pressure will be consistently offset from the true average. This results in an error in the prediction of air flow at a steady state operating condition. Another disadvantage, for example, is that the resulting averaged manifold pressure will still contain oscillations that will cause cyclic errors in prediction of air flow at a steady state operating condition. These cyclic errors may cause reduced efficiency in controlling regulated emissions.
Also, engine control systems relying on a manifold pressure sensor to determine fresh charge entering the engine must be able to measure flow of exhaust gas recirculation to accurately control the exhaust air/fuel ratio. Previous systems have used a differential pressure measurement across an orifice to infer a flow of exhaust gas. Traditionally, the orifice is located upstream of the exhaust gas recirculation flow control valve. Thus, the pressure measurements are shielded from the intake manifold pressure pulsations; however, the pressure measurements are not shielded from the exhaust pressure pulsations. In the traditional system, the high frequency pressure pulsations present in the pressure measurements are reduced by using a conventional low pass filter. Such a system is disclosed in U.S. Pat. No. 5,613,479.
The inventors herein have recognized a significant opportunity to reduce total system cost by relocating the orifice downstream of the exhaust gas recirculation flow control valve but before the intake manifold. Thus, the manifold pressure sensor can be used to measure the pressure downstream of the orifice and a single absolute pressure sensor can be used to measure the pressure upstream of the orifice. This creates the needed differential pressure to measure exhaust gas recirculation flow.
The inventors herein have recognized numerous disadvantages with the above approach. For example, the manifold pressure sensor is sensitive to pressure fluctuations in the manifold and the upstream exhaust pressure sensor is sensitive to pressure fluctuations in the exhaust pressure. Since these fluctuations are out of phase with one another, a significant error is created in the difference between the two. Another disadvantage is the need for a conventional low pass filter to reduce these oscillations, where the conventional low pass filter is known to hinder transient performance.